Friday, March 13, 2009

Exposure Calibration

If you are uncertain of how much time is required to reach full exposure with your light source, obtain a 21 step Stouffer Sensitivity Guide or "step tablet" from your local camera shop and follow the instructions included with the soldermask or photoresist. A Stouffer step tablet consists of a series of 21 steps which vary in density from totally clear (step 1) to totally opaque (step 21). For the UV wavelengths used in printed circuit applications, the ratio of effective exposure between any two consecutive steps is a constant factor of about 1.414 (i.e. square root of 2, or ½ "f" stop).

To calibrate your light source:

  • Laminate a piece of substrate with the soldermask (or photoresist) that you intend to use.
  • Position the 21 step Stouffer gauge between the soldermask (or photoresist) and a clear area on your artwork. Be certain that nothing on the artwork overlaps the step tablet.
  • Put the ensemble into a vacuum frame (if available) and draw it down.
  • Calculate an initial exposure time by dividing the power output of your UV lamp (in Watts) by the total area being illuminated (in cm2). Divide the result into the total energy requirements of the dry-film (in miliJoules/cm2). Multiply the result by 2. The final number that you come up with will be a pretty good first estimate of the needed exposure time in seconds.
  • Expose your "board".
  • Let the board sit for 15 - 30 min., strip off the Mylar cover sheet and develop as described below.
  • When the pattern is completely developed, examine the image of the Stouffer gage. The highest number step that still shows some material present after developing is your "exposure number". The highest number that shows no removal of film is referred to as the "step held". In the case of DF-8030 Soldermask, the "exposure number" should fall between 9 and 12. For DF 4615 Photoresist, the "exposure number" should fall between 7 and 10.

If you get an initial exposure number of 5 (as shown, step held = 4), you should increase the exposure time five full steps to reach the center of the recommended range (10 for Vacrel 8020); i.e. multiply by a factor of; 1.414 * 1.414 * 1.414 *1.414 *1.414 =5.66.

In other words, multiply your initial exposure time by 5.66, and run the test again.

  • If the initial exposure number is too high, your board is over exposed. Count back to the center of the desired range and divide your exposure time by the appropriate divisor.

If you get an initial number of 13, calculate the proper exposure by dividing your initial time by:
Exposure = (initial time)/(1.414 *1.414 * 1.414) or Exposure = (initial time) / (2.83)

It may require as many as three test exposures to "zero" the process in but, once you have determined a working exposure time, it will always be a good starting point, even when your bulbs age or need replacement. This calibration should be conducted on every photoimageable material you will be using and the results recorded in your dated process log (Don't keep one? Now is a good time to start.)

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